Thermochemical Calculations

CA Standards

Students know energy is released when a material condenses or freezes and is absorbed when a material evaporates or melts.

Students know how to solve problems involving heat flow and temperature changes, using known values of specific heat and latent heat of phase change.

Units for Measuring HeatThe Joule is the SI system unit for measuring heat:

The calorie is the heat required to raise the temperature of 1 gram of water by 1 Celsius degree

2

2111

s

mkgmeternewtonJoule

Joulescalorie 18.41

Specific Heat

The amount of heat required to raise the temperature of one gram of substance by one degree Celsius.

1

2

3

45 6

7

8

9

1 10

2

3

45 6

7

8

9

11

Calculations Involving Specific Heat

cp = Specific Heat

Q = Heat lost or gained

T = Temperature change

OR

m = Mass

Tm

Qcp

pcTmQ

Specific HeatThe amount of heat required to raise the temperature of one gram of substance by one degree Celsius.Substance Specific Heat (J/g·K)

Water (liquid) 4.18

Ethanol (liquid) 2.44

Water (solid) 2.06

Water (vapor) 1.87

Aluminum (solid) 0.897

Carbon (graphite,solid) 0.709

Iron (solid) 0.449

Copper (solid) 0.385

Mercury (liquid) 0.140

Lead (solid) 0.129

Gold (solid) 0.129

Latent Heat of Phase Change

Molar Heat of Fusion

The energy that must be absorbed in order to convert one mole of solid to liquid at its melting point.

The energy that must be removed in order to convert one mole of liquid to solid at its freezing point.

Molar Heat of Solidification

Latent Heat of Phase Change #2

Molar Heat of VaporizationThe energy that must be absorbed in order to convert one mole of liquid to gas at its boiling point.

The energy that must be removed in order to convert one mole of gas to liquid at its condensation point.

Molar Heat of Condensation

Latent Heat – Sample Problem

Problem: The molar heat of fusion of water is6.009 kJ/mol. How much energy is needed to

convert 60 grams of ice at 0C to liquid water at 0C?

Massof ice

MolarMass ofwater

Heatof

fusion

kiloJoulesOHmol

kJ

OHg

OHmolOHg20

1

009.6

02.18

160

22

22

Heat of SolutionThe Heat of Solution is the amount of heat energy absorbed (endothermic) or released (exothermic) when a specific amount of solute dissolves in a solvent.

Substance

Heat of Solution (kJ/mol)

NaOH -44.51

NH4NO3 +25.69

KNO3 +34.89

HCl -74.84